Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q13/00—Waveguide horns or mouths; Slot antennas; Leaky-waveguide antennas; Equivalent structures causing radiation along the transmission path of a guided wave
  • H01Q13/20—Non-resonant leaky-waveguide or transmission-line antennas; Equivalent structures causing radiation along the transmission path of a guided wave
  • H01Q13/206—Microstrip transmission line antennas
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
  • G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
  • G01S13/04—Systems determining presence of a target
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
  • G01S7/35—Details of non-pulse systems
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q21/00—Antenna arrays or systems
  • H01Q21/06—Arrays of individually energised antenna units similarly polarised and spaced apart
  • H01Q21/061—Two dimensional planar arrays
  • H01Q21/065—Patch antenna array
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
  • H01Q9/04—Resonant antennas
  • H01Q9/0407—Substantially flat resonant element parallel to ground plane, e.g. patch antenna
  • H01Q9/0428—Substantially flat resonant element parallel to ground plane, e.g. patch antenna radiating a circular polarised wave
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
  • H01Q9/04—Resonant antennas
  • H01Q9/0407—Substantially flat resonant element parallel to ground plane, e.g. patch antenna
  • H01Q9/0464—Annular ring patch

Definitions

• the present invention generally relates to the technical field of sensing objects internal or external to a vehicle, and more particularly to a narrowband leaky wave antenna with large beamsweep for radar sensing in relation to a vehicle.
• OD occupancy detection
• OC occupancy classification
• SBR seat- belt reminder
• Radar technology offers some advantages in comparison to other occupancy detection and classification methods. It is a contactless and invisible measurement system which can easily be integrated behind plastic covers and textiles. Radar systems can measure the smallest motions - within the range of micrometers, e.g. using microwaves.
• the radar system uses an antenna with frequency dependent radiation directions (e.g. a kind of a leaky wave antenna or meta material antenna), so the antenna can sense into different directions. By observing the motion in the pure CW signal the angle of motion maximum can be determined. By use of an advanced autocalibration method the motion can be extracted and classified. This allows to distinguish between objects with arbitrary motions (due to driving situations) and humans/animals inhering a breathing motion of the chest. By scanning over the surface of the human/animal and interpreting the reflected motion signal the size of the human can be estimated which allows a classification.
• frequency dependent radiation directions e.g. a kind of a leaky wave antenna or meta material antenna
• JUHUA LIU ET AL "Design and Analysis of a Low-Profile and Broadband Microstrip Monopolar Patch Antenna", IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, IEEE SERVICE CENTER, PISCATAWAY, NJ, US, vol. 61 , no. 1 , 1 January 2013 (2013-01 -01 ), pages 1 1 -18, XP01 1484563, ISSN: 0018- 926X, discloses a microstrip monopolar patch antenna with a wide bandwidth and a monopole like radiation pattern.
• Such antenna is constructed as a circular patch (disc-shaped) antenna of radius R. The antenna is shorted concentrically with a set of (e.g.
• the antenna can operate as a broadband monopolar patch antenna that utilizes two modes (TM01 and TM02 modes).
• US3987455A discloses a microstrip antenna having one or more arrays of resonant dipole radiator elements.
• the radiator elements have an E coordinate dimension of approximately In embodiments, the radiator elements
• the elliptical radiator elements each have an E coordinate dimension of 1 .52 cm, an H coordinate dimension of 2.03 cm and are located on 2.54 cm centers.
• a problem is that currently known leaky wave antenna systems are designed by Q-balancing method and an asymmetric approach in order to achieve circular polarization. Frequency sensitive beams and simultaneously small beams are rarely catered for, if at all.
• An issue is that common optimization methods employ only the Q-balancing condition and not all necessary parameters like mutual coupling, input reflection coefficient, axial ratio and open stopband suppression.
• the Q- factor of the patches employed needs to be as high as possible.
• a circular patch or cell may be used as radiating element, as the ratio between circumference and the area is best for circles.
• the E01 mode may be used for radiation. This mode is excited by the feeding network.
• the patch antenna may be used itself to rotate the field and shift the phase for the next unit cell.
• an antenna element comprising a cell of filled circular geometry adapted to operate, when driven in use, in the E01 mode.
• the antenna element may further include a stub configuration adapted to achieve a matched network using a mode rotation condition, wherein said cell is coupled to a feed line at a first point on a perimeter of the cell such that the feed line extends tangentially to the cell.
• the antenna element may further include a delay loop configured for mode rotation.
• the delay loop may be adapted to achieve Q-balancing, mutual coupling compensation and/or tuning the reflection coefficient as well as an axial ratio close to one within the delay loop length.
• the antenna element is configured to be coupled to a feed line at a first point on a perimeter of the cell, and the delay loop is attached to a side of the cell at points on the perimeter substantially opposite to the side containing the first point.
• a y-direction is defined by or parallel to a line joining the center of the cell with said first point, and a line through said center and perpendicular to the plane of the cell defines an x-direction; and wherein the delay loop is symmetrical in the x-y plane.
• a z-direction is defined through said center and perpendicular to the x-y plane.
• the antenna element may be in a form allowing the first spatial mode in z- direction to radiate.
• the antenna element may further comprise a passive spatial mode filter.
• the passive spatial mode filter comprises the delay loop with integrated delay lines.
• the passive spatial mode filter comprises an upper half circle and a lower half circle, with a respective delay line coupling each pair of opposing ends of the upper half circle and a lower half circle.
• a unit cell comprising a cell as substantially disclosed herein above, a delay loop and a feed line portion, the feed line portion being an element of the feed line.
• the parameters of the unit cell are given by the right hand column of Table 4.1 hereinbelow.
• a leaky wave antenna comprising a plurality of antenna elements or a plurality of unit cells as disclosed above.
• the antenna elements may be connected to each other in a linear array, so as to form a series fed patch antenna.
• the leaky wave antenna may be of periodic form.
• the antenna elements are preferably connected to a feed line.
• the passive spatial mode filter may comprise a filter loop attached at points on a side of the cell opposite to the point of connection of the cell to the feed line.
• a phase shift within each unit cell is 2 ⁇ where n is an integer.
• the parameters of the leaky wave antenna are given by the right hand column of Table 4.2 hereinbelow.
• the leaky wave antenna may be manufactured using microstrip technology on a printed circuit board (PCB).
• PCB printed circuit board
• a radar sensor system for sensing occupancy status within an automotive vehicle, the system comprising: an antenna system, for illuminating at least one occupiable position within the vehicle with continuous wave (CW signals), the CW signals being frequency modulated in time, the antenna system comprising a leaky wave antenna as disclosed above;
• CW signals continuous wave
• the antenna system comprising a leaky wave antenna as disclosed above;
• At least one sensor for receiving at least one sensor signal reflected as a result of the CW signals
• processing circuitry coupled to the at least one sensor, the processing circuitry being operable for applying, occupancy detection algorithms to the at least one sensor signal to generate a modified signal; and generating, based on the modified signals, one or more occupancy status signals, the occupancy status signal indicating a property related to said at least one occupiable position.
• the antenna system may be adapted to illuminate with a steerable beam from -45° to 45° and/or a steerable beam from -5° to 5° in the 24 GHz ISM Band.
• a large beamsweep over the frequency is provided, as well as a small beam, in order to achieve high angle resolution.
• Small beams are possible as the structure is elongate. Therefore, a series fed patch leaky wave antenna is preferably employed. This type of antenna is less beam sensitive compared to a meta-material approach. A high quality factor affects a more frequency sensitive beam. Therefore, a round/circular patch geometry is used, which also affords good axial ratio properties with respect to circular polarisation. Within the delay loop and the circular patch stub configuration, the reflection coefficient, axial ratio, mutual coupling effects and open stopband suppression can be done by tuning just one parameter.
• the radar sensor system is provided for in-vehicle occupant detection and classification (e.g. airbag suppression), for passenger presence detection, for passenger's vital sign monitoring and/or for seat-belt reminder functionality (SBR).
• the system includes an antenna system which illuminates one or more seats of an automotive vehicle.
• Advantages of the invention include: only one sensor is necessary for the whole rear bench of an automobile (one single RF channel with l/Q output);
• Fig. 1 is a schematic diagram of sensor positions within a passenger compartment of a vehicle for an embodiment of the invention
• Fig. 2 shows the nomenclature of polar coordinates for circular patch, according to an embodiment of the invention
• Fig. 3 shows the first kind Bessel function for orders zero to two
• Fig. 4 shows the electric boundary condition in propagation direction, according to an embodiment of the invention
• Fig. 5 shows a unit cell with filter loop, according to an embodiment of the invention
• Fig. 6 shows a schematic view of an embodiment of a unit cell according to the invention
• Fig. 7 shows a unit cell's even and odd excitation, according to an embodiment of the invention.
• Fig. 12 shows a simulated unit cell scattering parameter amplitude, according to an embodiment of the invention
• Fig. 13 shows a simulated unit cell scattering parameter phase, according to an embodiment of the invention
• Fig. 14 shows a simulated improved unit cell scattering parameter amplitude, according to an embodiment of the invention
• Fig. 15 shows a simulated improved unit cell scattering parameter phase, according to an embodiment of the invention.
• Fig. 20 shows a unit cell dispersion relation, according to an embodiment of the invention.
• Fig. 21 shows an improved unit cell dispersion relation, according to an embodiment of the invention.
• Fig. 22 shows scattering parameter amplitude for a 15 cell array, according to an embodiment of the invention
• Fig. 23 shows an improved 15 cell array 3D farfield radiation pattern, according to an embodiment of the invention.
• Fig. 26 shows a schematic view of an embodiment of a unit cell
• Fig. 29 shows a scattering parameter amplitude for an improved 15 cell array
• Fig. 30 shows an array dispersion relation - improved (solid line) and not improved (dashed line), according to an embodiment of the invention
• Fig. 31 shows an array electric field distribution in even mode excitation
• Fig. 32 shows an array electric field distribution in odd mode excitation
• Fig. 33 shows an azimuth radiation pattern for broadside radiation, according to an embodiment of the invention.
• Fig. 34 shows an array field distribution beyond centre frequency, according to an embodiment of the invention.
• Fig. 35 shows an azimuth radiation pattern for positive endfire radiation, according to an embodiment of the invention.
• Fig. 36 shows an azimuth radiation pattern for negative endfire radiation, according to an embodiment of the invention
• Fig. 37 shows a 25 cell leaky wave antenna, according to an embodiment of the invention.
• Fig. 38 shows the scattering parameter amplitude for a 25 cell simulated Array under real conditions, according to an embodiment of the invention
• Fig. 39 shows a dispersion relation of a 25 cell simulated array under real conditions, according to an embodiment of the invention.
• Fig. 40 shows a measured scattering parameter amplitude of a circular patch leaky wave antenna, according to an embodiment of the invention
• Fig. 41 shows a measured dispersion relation of a circular patch leaky wave antenna, according to an embodiment of the invention.
• Fig. 42 shows the beam steering capability within the 24GHz ISM band, according to an embodiment of the invention.
• Fig. 43 shows the beam steering capability within the frequency range 23GHz to
• Fig. 44 shows the beam steering capability within the frequency range 23GHz to
• Fig. 45 shows radiation pattern of the design of [6]
• Fig. 46 shows reflection coefficient of the design of [6]
• Fig. 47 shows a coordinate system definition according to known leaky wave antenna technology
• Fig. 48 shows a unit cell leaky wave transmission line model according to known leaky wave antenna technology
• Fig. 49 shows the Bloch wave transmission line model according to known leaky wave antenna technology
• Fig. 50 shows a unit cell lattice model according to known leaky wave antenna technology
• Fig. 51 shows the azimuth leaky wave radiation pattern parameter according to known leaky wave antenna technology
• Fig. 52 shows a dispersion diagram according to known leaky wave antenna technology.
• radar systems in contrast to passive camera systems the radar system operates also in the night as it illuminates actively the scene. Active camera systems need an illumination in the light spectrum which can be visible for the human eye (compare to the glooming of infrared cameras). Radar systems work in the microwave range which is completely invisible for the human eye. In addition it penetrates into materials and can transmit through. So the integration behind plastic covers and textiles as well is possible while camera based systems need an opening to look through.
• FIG. 1 is a schematic diagram of sensor positions within a passenger compartment of a vehicle for an embodiment of the invention.
• the position of sensor installation is preferably in front of the occupant, so that the whole area of the body or a part of it is illuminated.
• Possible integration places can be between top ceiling and car body, in the car body pillars, center console, overhead console, dash boards, steering wheel etc.
• Figure 1 (a) shows a radar sensor 10 built into the ceiling 12 of a vehicle.
• the sensor 10 includes at least one antenna (not shown) for illuminating one, two or three of the (occupiable) seating positions on the rear bench 14 of the vehicle, for detecting the presence of one or more occupants 1 1 .
• Figure 1 b shows a radar sensor 10' for sensing occupancy in the front of the vehicle.
• the sensor 10' is built into front console 16, and includes at least one antenna (not shown) for illuminating one or both of the front seats of the vehicle.
• the senor 10, 10' may additionally include transm it-receive electronics and processing circuitry for carrying out the occupancy detection operations as discussed herein below, or the latter may be coupled to the antenna in sensor 10, 10', but located elsewhere within the vehicle.
• Embodiments of the invention relate to leaky wave antennas.
• the advantage of leaky wave antennas is their frequency steerable radiation beam capability.
• Embodiments of the invention relate to two leaky wave antennas - one with a steerable beam from -45° to 45° and the other from -5° to 5° in the 24 GHz ISM Band.
• Embodiments of the invention utilize the whole frequency band for the given angle range.
• Embodiments of the invention are manufactured with printed circuit board technology.
• Embodiments of the invention comprise periodic leaky wave antennas are investigated.
• Embodiments of the invention relate to leaky wave antennas which are used for interior automotive applications with reduced cost. Embodiments of the invention relate to the use of three antennas in order to achieve a separation of three persons on the rear bench. Because of the beam steering capability of leaky wave antennas with their narrow beam, embodiments of the invention relate to should render it easier to distinguish multiple persons as well as reduction in the number of sensors from three to one.
• leaky wave antennas The concept of leaky wave antennas is to transform a three dimensional radiation problem into a two or especially one dimensional problem in the case of periodicity.
• the radiating mode named leaky mode is a derivation from the dispersion relation. Therefore the distinction of leaky modes and wave propagation requires discussion.
• a leaky mode is a radiative power leakage from the major mode. If there are several modes which are able to propagate and it is assumed that the influence of the excitation is neglected, the utilization of the vectorial Helmholtz equation is necessary.
• the vector -A represents an arbitrary field vector and k 0 represents the free space propagation constant.
• k 0 represents the free space propagation constant.
• the derivation in y-direction is set to zero
• the spatial Fourier transform of equation 2.1 yields
• Figure 47 shows a coordinate system definition: as implied in Figure 47, the z-direction is the propagation along the transmission line and the x-direction is the radiation at broadside.
• the radiation condition for the lossless case can be derived from the dispersion relation [1 ].
• Equation 2.4 is the leakage condition and can be used for design considerations. In order to achieve the radiation condition, it is necessary to quantize the propagation constant From transmission line theory it is well
• Figure 48 shows a unit cell leaky wave transmission line model. The aim is to determine these lumped elements as well as the length of the unit cell.
• the bloch wave analysis uses the periodicity along the z-direction. This means that every unit cell's input and output are connected to the same unit cell. The interconnection between an infinite number of unit cells yields to a spatial harmonic in the z-direction.
• the theory of infinite periodic structures is based on Floquet's theorem [1].
• the electric field's longitudinal periodicity can be express as follows [1 ]:
• the periodic electric field can be expanded in a spatial Fourier series within the z-direction.
• a simple transmission line model which is a linearised version of the distributed structure within the centre frequency ⁇ .
• Figure 49 shows the Bloch wave transmission line model.
• the Bloch impedance can be determined by the following equation.
• the minus/plus operator in the denominator denotes on which port the Bloch impedance is calculated. It depends on the direction of counter arrows on the ports.
• Finite structures affect an odd excitation for the series resistance and an even excitation for the for the shunt admittance.
• Using a lattice model solves the problem of decoupling series impedance Z s and the shunt admittance Y p [2].
• Figure 50 shows a unit cell lattice model.
• the Dispersion diagram is the visualisation of the dispersion relation. Usually the propagation constant is even the abscissa and the frequency is the ordinate. Considering the lossless case, the propagation constant yields to be complex in a certain frequency band.
• the -3 dB beamwidth is a function of the attenuation constant, free space propagation constant and the main beam direction [6]:
• Fig. 51 shows the azimuth leaky wave radiation pattern parameter. In Fig. 51 it will be shown that all parameters which shape the radiation pattern are also depicted in the dispersion diagram shown in Fig. 52. Hence, the dispersion diagram is an appropriate tool to design leaky wave antennas. [0072] Furthermore, maximum and minimum main beam direction angles
• the attenuation constant becomes frequency independent if the heavy side condition or Q-balancing is acquired [8].
• the expression respectively is replaced under the restriction of open stopband suppression by
• the minimum and maximum radiation angle depends on unit cells Q-factor.
• Equation 2.1 1 The relationship of equation 2.1 1 can be expressed by transmission parameter and scattering parameter respectively [9].
• the terms "antenna element”, “patch”, “unit cell” and “cell” will be used interchangeably for an element making up a leaky wave antenna.
• the major issues of unit cell modelling addressed by the invention are the suppression of the open stopband and the achievement of high slope in the dispersion relation which implies a highly frequency sensitive beam.
• the unit cell according to an embodiment of the invention is in a form allowing the first spatial mode in z-directrion to radiate. Furthermore the unit cells may be connected to each other, e.g. linearly, which yields to a series fed patch design.
• Embodiments of the invention are based on series fed patch (SFP) leaky wave antenna.
• SFP series fed patch
• the design is accomplished by the cavity model [10].
• the radiated field can be calculated with equivalent magnetic currents.
• the magnetic current depends on the perimeter of microstrip geometry. If radiation is the only power loss in the structure the perimeter is proportional to the radiated power.
• the surface of microstrip geometry is a measure of bounded fields. Bounded fields imply stored energy, meaning that the definition of the Q-factor is simplified proportional to the ratio of area to perimeter.
• the simplification to the cavity model is valid. Furthermore, there is just one design parameter.
• the other parameters of interest such as the Q-factor and the radiation characteristic can be deduced from the radius. If the Q-factor and the radiation characteristic are known, a connection between modes of a circular cavity and lumped elements is possible.
• Figure 2 depicts the top view of a circular patch according to an embodiment of the invention.
• the circle has an effective radius rO and constitutes a magnetic wall boundary condition.
• the patch is manufactured using printed circuit board technology, the top and bottom of the patch are copper and are separated by the height of substrate h.
• the copper boundary is just an electric wall.
• the solution of the wave equation for this kind of cavity can be done by separating the variables [10].
• Figure 3 shows the first kind Bessel function for order zero to two.
• the relationship between the effective electric rO and the geometric rg can be determined by numerical fitted function [10].
• the radiated electric field can be used to determine the radiated power using its poynting vector. Integrating the poynting vector over a sphere, yields to the radiation conductance which can be numerically calculated [10].
• a wave is able to propagate along a structure if there is an excitation of series and shunt mode. Furthermore, for artificial transmission line segments, the balanced condition must be met.
• FIG. 50 the basic approach for unit cell design is the even and odd analysis.
• Even and odd analysis in mode considerations yields to a magnetic wall boundary in z-direction for shunt mode and electric wall boundary condition for series mode respectively [1 1 ].
• Figure 4 shows the electric boundary condition in the propagation direction, demonstrating the boundary condition for electric wall boundary condition because the magnetic boundary condition is still met by the cavity model.
• Figure 4 illustrates that the electric boundary condition forces a shunt mode. Taking account of the coordinate transformation the appropriate wave equation solution of the bounded electric field is:
• the excitation signal waveform is sinusoidal and a transversal electromagnetic wave. If these conditions are satisfied, the complex source amplitude is equal to the complex mode amplitude at this point. This yields to a boundary condition in ⁇ -direction.
• Equation 3.20 has an infinite number of solutions, but the law of energy conversation yields to the constraint that the point ⁇ is a global extremum.
• a general definition for a one dimensional standing wave is:
• Every standing wave can be interpreted as a superposition of forward and backward travelling wave with the same amplitude.
• the basic principle is to achieve destructive interference of backward propagating waves and constructive interference of forward propagating wave using passive boundary conditions. This principle will be described in more detail in the following section.
• Equation 3.28 is a condition for a propagating wave in ⁇ -direction which is very similar to a mode rotation. This rotation can be done by two different feeding ports but only one feed line is available. Regarding the derivation of the above condition, there is a degree of freedom. In case of equation 3.27, both Hilbert transforms has been added which implies cancelling the backward travelling wave. If the Hilbert transforms were subtracted the forward travelling will be cancelled. So this relationship is very similar to the Single-Sideband Modulation where the upper side-band implies positive rotation and the lower sideband implies negative rotation. For the sake of completeness the condition to suppress the upper sideband is:
• Embodiments of the invention involve filtering an undesired mode in a passive structure which has a single feed.
• the structure may be transversal symmetric. This means the unit cell is symmetric with respect to the xy-plane. So the feeding point has to be in the symmetry plane.
• a loop may" be constructed.
• FIG. 5 shows an embodiment of an antenna element (unit cell) 2 comprising a filled circular cell (patch) 4 with filter loop 6.
• cell 4 and filter loop 6 are formed of copper on a dielectric substrate (not shown, using e.g. PCB manufacturing techniques.)
• the cell 4 is configured to be attached at point 8 in a stub configuration to a feed line (not shown).
• Using the symmetry condition and inserting the parameter of Figure 5 in equation 3.28 yields to the calculation 3.31 .
• the attenuation is very low and the amplitude will be approximately constant.
• the excitation position constitutes the origin.
• phased array radiates at broadside if all antennas are in phase. According to embodiments of the leaky wave antenna design, this means that it is necessary that the phase shift within each unit cell is 2 ⁇ where n is an integer. This matches with the results of Section 2 hereinabove to excite the nth mode at the resonant frequency.
• FIG. 6 schematically shows a schematic view of an embodiment of a unit cell 2'.
• the unit cell 2' is attached to a portion 10 of feed line of length p.
• the (filter) loop 6' comprises a delay loop formed by upper half circle 12, lower half circle 14 and straight line lengths 16, 18 of conductor.
• a first length 16 couples first end 20 of upper half circle 12 with first end 24 of lower half circle 14, and second length 18 couples second end 22 of upper half circle 12 with second end 26 of lower half circle 14.
• the unit cell period is even the effective wavelength he approach is to design a straight line with a wave impedance of 70 ⁇ and a length of and the circle above the line. It is not necessary that the loop wave impedance is matched because matched means there is no standing wave. This is adjusted by the rotation condition. So the impedance is set to 70 ⁇ . Using formulation in [10] the width and the corresponding effective length can be determined. At this point, the unit cell structure is completely determined.
• Table 3.1 shows values of the analytic calculated antenna parameter.
• the feeding line has to satisfy the certain electric and magnetic wall boundary conditions in order to achieve an even and odd mode excitation. In the following chapter this issue will be discussed in more detail.
• Embodiments of the unit cell design are based on the cavity model. In order to address coupling effects, a full wave improvement is preferably done, as discussed hereinafter. First, the unit cell according to an embodiment and the aforementioned rotation condition are verified with respect to the even odd analysis. Later, the unit cell considerations are expanded to an array structure and mutual coupling effects are discussed. Finally, the transmission line characteristics as discussed in Section 2 hereinabove are addressed and are compared within the simulated farfield as well as within the measurement data.
• the structure of the unit cell 2' is schematically depicted in Fig. 6.
• the structure consists of three elements: a delay loop 6', a feeding line (portion) 10 and the resonant circular patch 4.
• the delay loop guides a propagating wave and the field inside the circle rotates in a mathematical positive manner. Coupling effects and a small radius of the delay loop influence the real electrical length. So the length of delay loop has to be adjusted as well as the length of feeding line.
• the electrical length of the feeding line is influenced by capacitive coupling with respect to the resonant circular patch. The capacitive coupling also influences the resonant frequency of the patch.
• the quality factor balancing condition is the even the rotation condition.
• a travelling wave appears by even and odd considerations, as depicted in Fig. 7.
• Figure 7 demonstrates graphically a unit cell's even and odd excitation and how a travelling wave condition is related to this description. Furthermore, the even and odd modes are equivalent to orthogonal modes.
• Figs 8-10 the vertical lines are the unit cell boundaries and should be equal to the electrical length of the bounded field.
• the electric field Ex in Fig. 7 represents just the field distribution in the feeding line. If the boundary conditions for even and odd excitation are met, there are three electrical symmetry planes which have to be satisfied. These planes are the inception, the end and the middle of the unit cell. In even mode, these planes are electrical wall boundaries and for odd mode excitation the symmetry planes are magnetic walls. [00136]
• the comparison of Figs 8-10 shows that there is no orthogonal mode excitation. In the circular patch, only the even mode excited, so a standing wave appears. This standing wave phenomena is the aforementioned open stopband problem.
• FIG. 12 shows a simulated unit cell scattering parameter amplitude.
• the electrical wave length is influenced which can be seen in the phase information of the transmission factor.
• Figure 13 shows a Simulated Unit Cell Scattering Parameter Phase
• phase information and the field distribution yields to the right handed rotating field as predicted in the unit cell modelling.
• the delay loop guides a travelling wave which can be verified by the virtual middle symmetry plane.
• the dispersion relation is a powerful tool to verify the Q-balancing as well as the broad side radiation issue.
• the open stopband effect was discussed.
• the open stopband affects an increasing attenuation constant and a nonlinear propagation constant respectively.
• this effect can be seen for normalized frequencies above 1 .01 . That is the reason that in the previous subsection the field distribution of 25GHz is depicted where the open stopband effect appears.
• Q-balancing can be improved by adaptation of delay loop length.
• the normalized frequency intercept corresponds to the zero phase point of the transmission factor. So the intercept can be adjusted by the adaptation of the feeding line length.
• Figure 21 shows the dispersion relation after improving the design parameter. Q-balancing is achieved and the attenuation constant is flat in a wide frequency range. The propagation constant is approximately linear and the intercept is near the normalized frequency.
• the next step is the expansion from a unit cell to an array structure under the condition that 90% of the input power is radiated.
• a big problem with array structures is mutual coupling effects.
• Another important design parameter is the degree of circular polarisation, which can be measured by the axial ratio of far field.
• Figure 22 shows the scattering parameter amplitude for a 15 cell array.
• optimization can be done by the variation of the delay loop length as well as the variation of feeding line length.
• the adaptation of feeding line length is necessary because the sum of an amount of modes with different amplitudes and phases yields a phase shift in the resulting mode. This unnecessary phase shift can be compensated by length adaptation.
• the farfield axial ratio which describes the ratio of main axis of the electrical field.
• the ratio is 1 , or OdB.
• Figure 23 shows the radiation pattern of the improved array structure.
• Figure 24 shows an axial ratio for a 15 cell array with Figure 25
• Figure 26 shows a schematic view of a unit cell according to an embodiment of the invention.
• the other tuning parameter is the length I in-between the upper and lower half circle, in order to adapt the electrical length. After full wave simulation the variation yields the following new parameter set (Table 4.2) for the array structure.
• the reflection coefficient may be optimized, such that the delay loop has a different parameter for each unit cell, as in the middle of the structure higher coupling effects occur than at the ends of the structure.
• Figure 29 shows scattering parameter amplitude for an improved 15 cell array.
• the dispersion relation demonstrates the improvement of the above described optimization.
• Figure 30 shows an array dispersion relation - Improved (solid line) and not improved (dashed line).
• Figures 31 to 32 illustrate the electric field distribution for even and odd mode excitation, respectively.
• the figures below show that every field distribution is equal to each other and that the unit cells are in phase.
• Figure 34 shows an array field distribution beyond centre frequency. The variation of centre frequency to 24GHz yields a phase shift from one unit cell to each other. It is shown in Figure 34 that the rotation angle is continuously shifted to a mathematical positive direction.
• Figure 35 shows an azimuth radiation pattern for positive endfire radiation.
• a mathematical positive shift for instance in coordinate transformation yields to a negative sign in the argument. So the propagation constant becomes a negative value and, based on the phased array theory, the main beam direction becomes positive as depict in Figure 35.
• the same argumentation with inverse algebraic signs is valid for the negative endfire radiation, as shown in Figure 36.
• leaky wave antenna 40 is a series fed antenna formed by multiple unit cells 2, 2' formed in copper on substrate 42, with each cell 2, 2' being attached to feed line 44.
• the simulation was done by full wave simulation using CST microwave studio.
• the de-embedding reference plane is similar to the measurement reference plane.
• the simulation was done by frequency solver using copper as well as lossy substrate.
• the boundary conditions are selected as open add space (perfectly matched layer including air space).
• Figure 38 shows the scattering parameter amplitude for a 25-cell simulated array under real conditions.
• Figure 39 shows the dispersion relation for a 25-cell simulated array under real conditions.
• the transmission parameter demonstrates a homogeneous and approximately flat attenuation constant, as depicted in Figure 39.
• the measurement shows that the reflection coefficient is smaller than the simulation.
• Figure 40 shows a measured scattering parameter amplitude for a circular patch leaky wave antenna in accordance with an embodiment of the invention.
• Measurement faults with respect to less coupling may be due to the measurement environment, as well as a faulty de-embedding mechanism.
• Figure 41 shows a measured dispersion relation for a circular patch leaky wave antenna.
• the frequency range is from 22GHz to 25.5GHz within a relative bandwidth of about 15%.
• the following table uses the formulation of Section 2 hereinabove for the 3dB beam width and for the main lobe direction based on the measured data.
• the broadside radiation has a minor angle offset of 0.77°. And the beam width has a little degradation at the maximum, as well as at the minimum, radiation angle. It is to be noted that the equations in the above section 2 are approximate and may only be valid for broadside radiation. The calculated parameters in Table 4.3 may be verified by radiation pattern measurements.
• two leaky wave antennas are disclosed with a steering beam range from -45° to 45° and from -5° to 5°, respectively.
• the performance of the leaky wave antenna may have capability of steering a beam from approximately -1 ° to 2° in the given frequency range.
• Figure 42 shows the beam steering capability within the 24GHz ISM Band.
• the quality factor of the antenna is optimized.
• the consideration of the frequency range from 23GHz to 25GHz and the comparison with a series fed patch antenna azimuth radiation pattern of Figure 44 illustrates that the presented design has a more sensitive beam.
• both antennas can be compared.
• the sensitivity expression is normalized to the period length and multiplication with the speed of light.
• Figure 45 shows the radiation pattern of the design of [6] and Fig. 46 shows the reflection coefficient within the frequency.
• the normalized beam sensitivity s ⁇ of [6] is near the normalized sensitivity according to embodiments described herein.
• the embodiments described herein have a lower reflection peak.
• the Q-factor is influenced by * r .
• the height of the substrate is also a parameter which may have effects.
• the simplified expression for the propagation constant affords that the sensitivity can be improved by choosing a higher frequency.
• Using the 79GHz band improves the effective wavelength by 329%. With a relative bandwidth of 5%, it is maybe possible with the same structure to achieve an all over 1 10° beam steering. Considering interior automotive applications, the person to be detected would have to be within the antenna far field. So the antenna must be small and a length reduction when using a 79GHz approach, or meta material technique, is necessary.
• an effective design with a low number of parameters is presented.
• the low number of parameters makes it easy to optimize the structure.
• the structure has good transmission line characteristics over a broad frequency range.
• the present invention finds industrial application is systems such as automobile rear bench seatbelt reminders and child hyperthermia sensors.

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